biodiesel from micro algae

Biotechnology Advances xx (2007) xxx–xxx

+ MODEL

JBA-06071; No of Pages 13

www.elsevier.com/locate/biotechadv

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Research review paper

Biodiesel from microalgae

Yusuf Chisti ⁎

Institute of Technology and Engineering, Massey University, Private Bag 11 222, Palmerston North, New Zealand

Abstract

Continued use of petroleum sourced fuels is now widely recognized as unsustainable because of depleting supplies and thecontribution of these fuels to the accumulation of carbon dioxide in the environment. Renewable, carbon neutral, transport fuels arenecessary for environmental and economic sustainability. Biodiesel derived from oil crops is a potential renewable and carbonneutral alternative to petroleum fuels. Unfortunately, biodiesel from oil crops, waste cooking oil and animal fat cannot realisticallysatisfy even a small fraction of the existing demand for transport fuels. As demonstrated here, microalgae appear to be the onlysource of renewable biodiesel that is capable of meeting the global demand for transport fuels. Like plants, microalgae use sunlightto produce oils but they do so more efficiently than crop plants. Oil productivity of many microalgae greatly exceeds the oilproductivity of the best producing oil crops. Approaches for making microalgal biodiesel economically competitive withpetrodiesel are discussed.© 2007 Elsevier Inc. All rights reserved.

Keywords: Biofuels; Biodiesel; Microalgae; Photobioreactors; Raceway ponds

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 02. Potential of microalgal biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03. Microalgal biomass production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

3.1. Raceway ponds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 03.2. Photobioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

4. Comparison of raceways and tubular photobioreactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 05. Acceptability of microalgal biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 06. Economics of biodiesel production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07. Improving economics of microalgal biodiesel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

7.1. Biorefinery based production strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07.2. Enhancing algal biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 07.3. Photobioreactor engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

8. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0

⁎ Tel.: +64 6 350 5934; fax: +64 6 350 5604.E-mail address: [email protected].

0734-9750/$ - see front matter © 2007 Elsevier Inc. All rights reserved.doi:10.1016/j.biotechadv.2007.02.001

Please cite this article as: Chisti Y. Biodiesel from microalgae. Biotechnol Adv (2007), doi:10.1016/j.biotechadv.2007.02.001

mailto:[email protected]

http://dx.doi.org/10.1016/j.biotechadv.2007.02.001


Box 1

Biodiesel production

Parent oil used in making biodiesel consists oftriglycerides (Fig. B1) in which three fatty acidmolecules are esterified with a molecule of glycerol.In making biodiesel, triglycerides are reacted withmethanol in a reaction known as transesterification oralcoholysis. Transestrification produces methyl estersof fatty acids, that are biodiesel, and glycerol (Fig. B1).The reaction occurs stepwise: triglycerides are firstconverted to diglycerides, then tomonoglycerides andfinally to glycerol.

Fig. B1. Transesterification of oil to biodiesel. R1–3 arehydrocarbon groups.

Transesterification requires 3 mol of alcohol foreach mole of triglyceride to produce 1 mol of glyceroland 3 mol of methyl esters (Fig. B1). The reaction is anequilibrium. Industrial processes use 6mol ofmethanolfor eachmole of triglyceride (Fukuda et al., 2001). Thislarge excess of methanol ensures that the reaction isdriven in the direction of methyl esters, i.e. towardsbiodiesel. Yield of methyl esters exceeds 98% on aweight basis (Fukuda et al., 2001).

Transesterification is catalyzed by acids, alkalis(Fukuda et al., 2001; Meher et al., 2006) and lipaseenzymes (Sharma et al., 2001). Alkali-catalyzedtransesterification is about 4000 times faster thanthe acid catalyzed reaction (Fukuda et al., 2001).Consequently, alkalis such as sodium and potassiumhydroxide are commonly used as commercial catalystsat a concentration of about 1% by weight of oil.Alkoxides such as sodium methoxide are even bettercatalysts than sodium hydroxide and are beingincreasingly used. Use of lipases offers importantadvantages, but is not currently feasible because ofthe relatively high cost of the catalyst (Fukuda et al.,2001). Alkali-catalyzed transesterification is carriedout at approximately 60 °C under atmospheric pres-sure, as methanol boils off at 65 °C at atmosphericpressure. Under these conditions, reaction takes about90min to complete. A higher temperature can be usedin combination with higher pressure, but this isexpensive. Methanol and oil do not mix, hence thereaction mixture contains two liquid phases. Otheralcohols can be used, but methanol is the leastexpensive. To prevent yield loss due to saponification

2 Y. Chisti / Biotechnology Advances xx (2007) xxx–xxx

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1. Introduction

Microalgae are sunlight-driven cell factories thatconvert carbon dioxide to potential biofuels, foods,feeds and high-value bioactives (Metting and Pyne,1986; Schwartz, 1990; Kay, 1991; Shimizu, 1996,2003; Borowitzka, 1999; Ghirardi et al., 2000; Akker-man et al., 2002; Banerjee et al., 2002; Melis, 2002;Lorenz and Cysewski, 2003; Metzger and Largeau,2005; Singh et al., 2005; Spolaore et al., 2006; Walteret al., 2005). In addition, these photosynthetic micro-organisms are useful in bioremediation applications(Mallick, 2002; Suresh and Ravishankar, 2004; Kalinet al., 2005; Munoz and Guieysse, 2006) and asnitrogen fixing biofertilizers Vaishampayan et al.,2001). This article focuses on microalgae as a potentialsource of biodiesel.

Microalgae can provide several different types ofrenewable biofuels. These include methane produced byanaerobic digestion of the algal biomass (Spolaore et al.,2006); biodiesel derived from microalgal oil (Roessleret al., 1994; Sawayama et al., 1995; Dunahay et al., 1996;Sheehan et al., 1998; Banerjee et al., 2002; Gavrilescuand Chisti, 2005); and photobiologically producedbiohydrogen (Ghirardi et al., 2000; Akkerman et al.,2002; Melis, 2002; Fedorov et al., 2005; Kapdan andKargi, 2006). The idea of using microalgae as a source offuel is not new (Chisti, 1980–81; Nagle and Lemke,1990; Sawayama et al., 1995), but it is now being takenseriously because of the escalating price of petroleumand, more significantly, the emerging concern aboutglobal warming that is associated with burning fossilfuels (Gavrilescu and Chisti, 2005).

Biodiesel is produced currently from plant andanimal oils, but not from microalgae. This is likely tochange as several companies are attempting to com-mercialize microalgal biodiesel. Biodiesel is a provenfuel. Technology for producing and using biodiesel hasbeen known for more than 50 years (Knothe et al., 1997;Fukuda et al., 2001; Barnwal and Sharma, 2005;Demirbas, 2005; Van Gerpen, 2005; Felizardo et al.,2006; Kulkarni and Dalai, 2006; Meher et al., 2006). Inthe United States, biodiesel is produced mainly fromsoybeans. Other sources of commercial biodieselinclude canola oil, animal fat, palm oil, corn oil, wastecooking oil (Felizardo et al., 2006; Kulkarni and Dalai,2006), and jatropha oil (Barnwal and Sharma, 2005).The typically used process for commercial production ofbiodiesel is explained in Box 1. Any future productionof biodiesel from microalgae is expected to use the sameprocess. Production of methyl esters, or biodiesel, frommicroalgal oil has been demonstrated (Belarbi et al.,

Please cite this article as: Chisti Y. Biodiesel from microalgae. Biotechnol Adv (2007), doi:10.1016/j.biotechadv.2007.02.001


Table 2Oil content of some microalgae

Microalga Oil content (% dry wt)

Botryococcus braunii 25–75Chlorella sp. 28–32Crypthecodinium cohnii 20Cylindrotheca sp. 16–37Dunaliella primolecta 23Isochrysis sp. 25–33Monallanthus salina N20Nannochloris sp. 20–35Nannochloropsis sp. 31–68Neochloris oleoabundans 35–54Nitzschia sp. 45–47Phaeodactylum tricornutum 20–30Schizochytrium sp. 50–77Tetraselmis sueica 15–23

Box 1 (continued )

reactions (i.e. soap formation), the oil and alcoholmustbe dry and the oil should have a minimum of free fattyacids. Biodiesel is recovered by repeatedwashingwithwater to remove glycerol and methanol.

3Y. Chisti / Biotechnology Advances xx (2007) xxx–xxx

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2000) although the product was intended for pharma-ceutical use.

2. Potential of microalgal biodiesel

Replacing all the transport fuel consumed in theUnited States with biodiesel will require 0.53 billion m3

of biodiesel annually at the current rate of consumption.Oil crops, waste cooking oil and animal fat cannotrealistically satisfy this demand. For example, meetingonly half the existing U.S. transport fuel needs bybiodiesel, would require unsustainably large cultivationareas for major oil crops. This is demonstrated inTable 1. Using the average oil yield per hectare fromvarious crops, the cropping area needed to meet 50% ofthe U.S. transport fuel needs is calculated in column 3(Table 1). In column 4 (Table 1) this area is expressed asa percentage of the total cropping area of the UnitedStates. If oil palm, a high-yielding oil crop can begrown, 24% of the total cropland will need to be devotedto its cultivation to meet only 50% of the transport fuelneeds. Clearly, oil crops cannot significantly contributeto replacing petroleum derived liquid fuels in theforeseeable future. This scenario changes dramatically,if microalgae are used to produce biodiesel. Between 1and 3% of the total U.S. cropping area would besufficient for producing algal biomass that satisfies 50%of the transport fuel needs (Table 1). The microalgal oilyields given in Table 1 are based on experimentally

Table 1Comparison of some sources of biodiesel

Crop Oil yield(L/ha)

Land areaneeded (M ha) a

Percent of existingUS cropping area a

Corn 172 1540 846Soybean 446 594 326Canola 1190 223 122Jatropha 1892 140 77Coconut 2689 99 54Oil palm 5950 45 24Microalgae b 136,900 2 1.1Microalgae c 58,700 4.5 2.5a For meeting 50% of all transport fuel needs of the United States.b 70% oil (by wt) in biomass.c 30% oil (by wt) in biomass.

Please cite this article as: Chisti Y. Biodiesel from microalgae. Biotechno

demonstrated biomass productivity in photobioreactors,as discussed later in this article. Actual biodiesel yieldper hectare is about 80% of the yield of the parent cropoil given in Table 1.

In view of Table 1, microalgae appear to be the onlysource of biodiesel that has the potential to completelydisplace fossil diesel. Unlike other oil crops, microalgaegrow extremely rapidly andmany are exceedingly rich inoil. Microalgae commonly double their biomass within24 h. Biomass doubling times during exponential growthare commonly as short as 3.5 h. Oil content in microalgaecan exceed 80% by weight of dry biomass (Metting,1996; Spolaore et al., 2006). Oil levels of 20–50% arequite common (Table 2). Oil productivity, that is themass of oil produced per unit volume of the microalgalbroth per day, depends on the algal growth rate and theoil content of the biomass. Microalgae with high oilproductivities are desired for producing biodiesel.

Depending on species, microalgae produce manydifferent kinds of lipids, hydrocarbons and othercomplex oils (Banerjee et al., 2002; Metzger andLargeau, 2005; Guschina and Harwood, 2006). Not allalgal oils are satisfactory for making biodiesel, butsuitable oils occur commonly. Using microalgae toproduce biodiesel will not compromise production offood, fodder and other products derived from crops.

Potentially, instead of microalgae, oil producingheterotrophic microorganisms (Ratledge, 1993; Ratledgeand Wynn, 2002) grown on a natural organic carbonsource such as sugar, can be used to make biodiesel;however, heterotrophic production is not as efficient asusing photosynthetic microalgae. This is because therenewable organic carbon sources required for growingheterotrophic microorganisms are produced ultimately byphotosynthesis, usually in crop plants.

l Adv (2007), doi:10.1016/j.biotechadv.2007.02.001


Fig. 1. Arial view of a raceway pond.


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Production of algal oils requires an ability toinexpensively produce large quantities of oil-richmicroalgal biomass.

3. Microalgal biomass production

Producing microalgal biomass is generally moreexpensive than growing crops. Photosynthetic growthrequires light, carbon dioxide, water and inorganic salts.Temperature must remain generally within 20 to 30 °C.To minimize expense, biodiesel production must rely onfreely available sunlight, despite daily and seasonalvariations in light levels.

Growth medium must provide the inorganic elementsthat constitute the algal cell. Essential elements includenitrogen (N), phosphorus (P), iron and in some casessilicon. Minimal nutritional requirements can beestimated using the approximate molecular formula ofthe microalgal biomass, that is CO0.48H1.83N0.11P0.01.This formula is based on data presented by Grobbelaar(2004). Nutrients such as phosphorus must be suppliedin significant excess because the phosphates addedcomplex with metal ions, therefore, not all the added P isbioavailable. Sea water supplemented with commercialnitrate and phosphate fertilizers and a few othermicronutrients is commonly used for growing marinemicroalgae (Molina Grima et al., 1999). Growth mediaare generally inexpensive.

Microalgal biomass contains approximately 50%carbon by dry weight (Sánchez Mirón et al., 2003).All of this carbon is typically derived from carbondioxide. Producing 100 t of algal biomass fixes roughly183 t of carbon dioxide. Carbon dioxide must be fedcontinually during daylight hours. Feeding controlled inresponse to signals from pH sensors minimizes loss ofcarbon dioxide and pH variations. Biodiesel productioncan potentially use some of the carbon dioxide thatis released in power plants by burning fossil fuels(Sawayama et al., 1995; Yun et al., 1997). This carbondioxide is often available at little or no cost.

Ideally, microalgal biodiesel would be carbon neutral,as all the power needed for producing and processing thealgae would come from biodiesel itself and frommethane produced by anaerobic digestion of biomassresidue left behind after the oils has been extracted.Although microalgal biodiesel can be carbon neutral, itwill not result in any net reduction in carbon dioxide thatis accumulating as a consequence of burning of fossilfuels.

Large-scale production of microalgal biomassgenerally uses continuous culture during daylight. Inthis method of operation, fresh culture medium is fed at


a constant rate and the same quantity of microalgalbroth is withdrawn continuously (Molina Grima et al.,1999). Feeding ceases during the night, but the mixingof broth must continue to prevent settling of the bio-mass (Molina Grima et al., 1999). As much as 25% ofthe biomass produced during daylight, may be lostduring the night because of respiration. The extent ofthis loss depends on the light level under which thebiomass was grown, the growth temperature, and thetemperature at night.

The only practicable methods of large-scale produc-tion of microalgae are raceway ponds (Terry andRaymond, 1985; Molina Grima, 1999) and tubularphotobioreactors (Molina Grima et al., 1999; Tredici,1999; Sánchez Mirón et al., 1999), as discussed next.

3.1. Raceway ponds

A raceway pond is made of a closed looprecirculation channel that is typically about 0.3 m deep(Fig. 1). Mixing and circulation are produced by apaddlewheel (Fig. 1). Flow is guided around bends bybaffles placed in the flow channel. Raceway channelsare built in concrete, or compacted earth, and may belined with white plastic. During daylight, the culture isfed continuously in front of the paddlewheel wherethe flow begins (Fig. 1). Broth is harvested behindthe paddlewheel, on completion of the circulation loop.The paddlewheel operates all the time to preventsedimentation.

Raceway ponds for mass culture of microalgae havebeen used since the 1950s. Extensive experience existson operation and engineering of raceways. The largestraceway-based biomass production facility occupies anarea of 440,000 m2 (Spolaore et al., 2006). This facility,



Fig. 3. A fence-like solar collector.


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owned by Earthrise Nutritionals (www.earthrise.com),is used to produce cyanobacterial biomass for food.

In raceways, any cooling is achieved only byevaporation. Temperature fluctuates within a diurnalcycle and seasonally. Evaporative water loss can besignificant. Because of significant losses to atmosphere,raceways use carbon dioxide much less efficiently thanphotobioreactors. Productivity is affected by contami-nation with unwanted algae and microorganisms thatfeed on algae. The biomass concentration remains lowbecause raceways are poorly mixed and cannot sustainan optically dark zone. Raceway ponds and other openculture systems for producing microalgae are furtherdiscussed by Terry and Raymond (1985).

Production of microalgal biomass for making biodie-sel has been extensively evaluated in raceway ponds instudies sponsored by the United States Department ofEnergy (Sheehan et al., 1998). Raceways are perceivedto be less expensive than photobioreactors, because theycost less to build and operate. Although raceways arelow-cost, they have a low biomass productivity com-pared with photobioreactors.

3.2. Photobioreactors

Unlike open raceways, photobioreactors permitessentially single-species culture of microalgae forprolonged durations. Photobioreactors have been suc-cessfully used for producing large quantities of micro-algal biomass (Molina Grima et al., 1999; Tredici, 1999;Pulz, 2001; Carvalho et al., 2006).

A tubular photobioreactor consists of an array ofstraight transparent tubes that are usually made of plas-tic or glass. This tubular array, or the solar collector, iswhere the sunlight is captured (Fig. 2). The solar col-lector tubes are generally 0.1 m or less in diameter. Tubediameter is limited because light does not penetrate toodeeply in the dense culture broth that is necessary forensuring a high biomass productivity of the photobior-eactor. Microalgal broth is circulated from a reservoir(i.e. the degassing column in Fig. 2) to the solar collector

Fig. 2. A tubular photobioreactor with parallel run horizontal tubes.


and back to the reservoir. Continuous culture operation isused, as explained above.

The solar collector is oriented to maximize sunlightcapture (Molina Grima et al., 1999; SánchezMirón et al.,1999). In a typical arrangement, the solar tubes areplaced parallel to each other and flat above the ground(Fig. 2). Horizontal, parallel straight tubes are sometimesarranged like a fence (Fig. 3), in attempts to increase thenumber of tubes that can be accommodated in a givenarea. The tubes are always oriented North–South(Fig. 3). The ground beneath the solar collector is oftenpainted white, or covered with white sheets of plastic

Fig. 4. A 1000 L helical tubular photobioreactor at MurdochUniversity, Australia. Courtesy of Professor Michael Borowitzka,Murdoch University.


http://www.earthrise.com


Table 3Comparison of photobioreactor and raceway production methods

Variable Photobioreactorfacility

Raceway ponds

Annual biomassproduction (kg)

100,000 100,000

Volumetric productivity(kg m−3 d−1)

1.535 0.117

Areal productivity(kg m−2 d−1)

0.048 a 0.035 b

0.072 c

Biomass concentrationin broth (kg m−3)

4.00 0.14

Dilution rate (d−1) 0.384 0.250Area needed (m2) 5681 7828Oil yield (m3 ha−1) 136.9 d 99.4 d

58.7 e 42.6 e

Annual CO2

consumption (kg)183,333 183,333

System geometry 132 parallel tubes/unit;80 m long tubes;0.06 m tube diameter

978 m2/pond; 12 mwide, 82 m long,0.30 m deep

Number of units 6 8

a Based on facility area.b Based on actual pond area.c Based on projected area of photobioreactor tubes.d Based on 70% by wt oil in biomass.e Based on 30% by wt oil in biomass.


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(Tredici, 1999), to increase reflectance, or albedo. A highalbedo increases the total light received by the tubes.

Instead of being laid horizontally on the ground,the tubes may be made of flexible plastic and coiledaround a supporting frame to form a helical coil tu-bular photobioreactors (Fig. 4). Photobioreactors suchas the one shown in Fig. 4 are potentially useful forgrowing a small volume of microalgal broth, for ex-ample, for inoculating the larger tubular photobior-eactors (Fig. 2) that are needed for producingbiodiesel. Other variants of tubular photobioreactorsexist (Molina Grima et al., 1999; Tredici, 1999; Pulz,2001; Carvalho et al., 2006), but are not widely used.Artificial illumination of tubular photobioreactors istechnically feasible (Pulz, 2001), but expensive com-pared with natural illumination. Nonetheless, artificialillumination has been used in large-scale biomassproduction (Pulz, 2001) particularly for high-valueproducts.

Biomass sedimentation in tubes is prevented bymaintaining highly turbulent flow. Flow is producedusing either a mechanical pump (Fig. 2), or a gentlerairlift pump.Mechanical pumps can damage the biomass(Chisti, 1999a; García Camacho et al., 2001, 2007;Sánchez Mirón et al., 2003; Mazzuca Sobczuk et al.,2006), but are easy to design, install and operate. Airliftpumps have been used quite successfully (Molina Grimaet al., 1999, 2000, 2001; Acién Fernández et al., 2001).Airlift pumps for use in tubular photobioreactors aredesigned using the same methods that were originallydeveloped for designing conventional airlift reactors(Chisti et al., 1988; Chisti and Moo-Young, 1988, 1993;Chisti, 1989). Airlift pumps are less flexible than me-chanical pumps and require a supply of air to operate.Periodically, photobioreactors must be cleaned and sani-tized. This is easily achieved using automated clean-in-place operations (Chisti and Moo-Young, 1994; Chisti,1999b).

Photosynthesis generates oxygen. Under high irradi-ance, themaximum rate of oxygen generation in a typicaltubular photobioreactor may be as high as 10 g O2 m

−3

min−1. Dissolved oxygen levels much greater than theair saturation values inhibit photosynthesis (MolinaGrima et al., 2001). Furthermore, a high concentrationof dissolved oxygen in combination with intense sun-light produces photooxidative damage to algal cells. Toprevent inhibition and damage, the maximum tolerabledissolved oxygen level should not generally exceedabout 400% of air saturation value. Oxygen cannot beremoved within a photobioreactor tube. This limits themaximum length of a continuous run tube before oxygenremoval becomes necessary. The culture must periodi-


cally return to a degassing zone (Fig. 2) that is bubbledwith air to strip out the accumulated oxygen. Typically, acontinuous tube run should not exceed 80 m (MolinaGrima et al., 2001), but the exact length depends onseveral factors including the concentration of the bio-mass, the light intensity, the flow rate, and the con-centration of oxygen at the entrance of tube.

In addition to removing the accumulated dissolvedoxygen, the degassing zone (Fig. 2) must disengage allthe gas bubbles from the broth so that essentially bubble-free broth returns to the solar collector tubes. Gas–liquidseparator design for achieving complete disengagementof bubbles, has been discussed (Chisti and Moo-Young,1993; Chisti, 1998). Because a degassing zone is gen-erally optically deep compared with the solar collectortubes, it is poorly illuminated and, therefore, its volumeneeds to be kept small relative to the volume of the solarcollector.

As the broth moves along a photobioreactor tube, pHincreases because of consumption of carbon dioxide(CamachoRubio et al., 1999). Carbon dioxide is fed in thedegassing zone in response to a pH controller. Additionalcarbon dioxide injection points may be necessary atintervals along the tubes, to prevent carbon limitation andan excessive rise in pH (Molina Grima et al., 1999).




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Photobioreactors require cooling during daylighthours. Furthermore, temperature control at night is alsouseful. For example, the nightly loss of biomass due torespiration can be reduced by lowering the temperatureat night. Outdoor tubular photobioreactors are effective-ly and inexpensively cooled using heat exchangers. Aheat exchange coil may be located in the degassingcolumn (Fig. 2). Alternatively, heat exchangers may beplaced in the tubular loop. Evaporative cooling by watersprayed on tubes (Tredici, 1999), can also be used andhas proven successful in dry climates. Large tubularphotobioreactors have been placed within temperaturecontrolled greenhouses (Pulz, 2001), but doing so isprohibitively expensive for producing biodiesel.

Selecting a suitable microalgal biomass productionmethod for making biodiesel requires a comparison ofcapabilities of raceways and tubular photobioreactors.

4. Comparison of raceways and tubularphotobioreactors

Table 3 compares photobioreactor and racewaymethods of producing microalgal biomass. This com-parison is for an annual production level of 100 tof biomass in both cases. Both production methodsconsume an identical amount of carbon dioxide(Table 3), if losses to atmosphere are disregarded.The production methods in Table 3 are compared foroptimal combinations of biomass productivity andconcentration that have been actually achieved inlarge-scale photobioreactors and raceways. Photobior-eactors provide much greater oil yield per hectarecompared with raceway ponds (Table 3). This is be-cause the volumetric biomass productivity of photo-bioreactors is more than 13-fold greater in comparison

Fig. 5. Microalgal biomass recovered from the culture broth byfiltration moves along a conveyor belt at Cyanotech Corporation(www.cyanotech.com), Hawaii, USA. Photograph by Terry Luke.Courtesy of Honolulu Star-Bulletin.


with raceway ponds (Table 3). Both raceway andphotobioreactor production methods are technicallyfeasible. Production facilities using photobioreactorsand raceway units of dimensions similar to those inTable 3 have indeed been used extensively in com-mercial operations (Terry and Raymond, 1985; MolinaGrima, 1999; Molina Grima et al., 1999; Tredici, 1999;Pulz, 2001; Lorenz and Cysewski, 2003; Spolaoreet al., 2006).

Recovery of microalgal biomass from the broth isnecessary for extracting the oil. Biomass is easilyrecovered from the broth by filtration (Fig. 5), cen-trifugation, and other means (Molina Grima et al.,2003). Cost of biomass recovery can be significant.Biomass recovery from photobioreactor cultured brothcosts only a fraction of the recovery cost for brothproduced in raceways. This is because the typicalbiomass concentration that is produced in photobior-eactors is nearly 30 times the biomass concentrationthat is generally obtained in raceways (Table 3). Thus,in comparison with raceway broth, much smallervolume of the photobioreactor broth needs to be pro-cessed to obtain a given quantity of biomass.

5. Acceptability of microalgal biodiesel

For user acceptance, microalgal biodiesel will needto comply with existing standards. In the United Statesthe relevant standard is the ASTM Biodiesel Standard D6751 (Knothe, 2006). In European Union, separatestandards exist for biodiesel intended for vehicleuse (Standard EN 14214) and for use as heating oil(Standard EN 14213) (Knothe, 2006).

Microalgal oils differ from most vegetable oils inbeing quite rich in polyunsaturated fatty acids withfour or more double bonds (Belarbi et al., 2000). Forexample, eicosapentaenoic acid (EPA, C20:5n-3;five double bonds) and docosahexaenoic acid (DHA,C22:6n-3; six double bonds) occur commonly in algaloils. Fatty acids and fatty acid methyl esters (FAME)with 4 and more double bonds are susceptible tooxidation during storage and this reduces their ac-ceptability for use in biodiesel. Some vegetable oilsalso face this problem. For example, vegetable oilssuch as high oleic canola oil contain large quantities oflinoleic acid (C18:2n-6; 2-double bonds) and linolenicacid (C18:3n-3; 3-double bonds). Although these fattyacids have much higher oxidative stability comparedwith DHA and EPA, the European Standard EN 14214limits linolenic acid methyl ester content in biodieselfor vehicle use to 12% (mol). No such limitation existsfor biodiesel intended for use as heating oil, but


http://www.cyanotech.com


Fig. 6. Microalgal biodiesel refinery: producing multiple productsfrom algal biomass.


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acceptable biodiesel must meet other criteria relatingto the extent of total unsaturation of the oil. Totalunsaturation of an oil is indicated by its iodine value.Standards EN 14214 and EN 14213 require the iodinevalue of biodiesel to not exceed 120 and 130 g iodine/100 g biodiesel, respectively. Furthermore, both theEuropean biodiesel standards limit the contents ofFAME with four and more double bonds, to a maxi-mum of 1% mol.

In view of the composition of many microalgal oils,most of them are unlikely to comply with the Europeanbiodiesel standards, but this need not be a significantlimitation. The extent of unsaturation of microalgal oiland its content of fatty acids with more than 4 doublebonds can be reduced easily by partial catalytichydrogenation of the oil (Jang et al., 2005; Dijkstra,2006), the same technology that is commonly used inmaking margarine from vegetable oils.

6. Economics of biodiesel production

Recovery of oil from microalgal biomass andconversion of oil to biodiesel are not affected by whetherthe biomass is produced in raceways or photobioreac-tors. Hence, the cost of producing the biomass is the onlyrelevant factor for a comparative assessment of photo-bioreactors and raceways for producing microalgalbiodiesel.

For the facilities detailed in Table 3, the estimated costof producing a kilogram of microalgal biomass is$2.95 and $3.80 for photobioreactors and raceways,respectively. These estimates assume that carbon dioxideis available at no cost. The estimation methods used havebeen described previously (Humphreys, 1991; MolinaGrima et al., 2003). If the annual biomass productioncapacity is increased to 10,000 t, the cost of productionper kilogram reduces to roughly $0.47 and $0.60 forphotobioreactors and raceways, respectively, because ofeconomy of scale. Assuming that the biomass contains30% oil by weight, the cost of biomass for providing aliter of oil would be something like $1.40 and $1.81 forphotobioreactors and raceways, respectively. Oil recov-ered from the lower-cost biomass produced in photo-bioreactors is estimated to cost $2.80/L. This assumesthat the recovery process contributes 50% to the cost ofthe final recovered oil. In comparison with this, during2006, crude palm oil, that is probably the cheapestvegetable oil, sold for an average price of $465/t, orabout $0.52/L.

In the United States during 2006, the on-highwaypetrodiesel price ranged between $0.66 and $0.79/L.This price included taxes (20%), cost of crude oil (52%),


refining expenses (19%), distribution and marketing(9%). If taxes and distribution are excluded, the averageprice of petrodiesel in 2006 was $0.49/L with a 73%contribution from crude oil and 27% contribution fromrefining.

Biodiesel from palm oil costs roughly $0.66/L, or35% more than petrodiesel. This suggests that theprocess of converting palm oil to biodiesel adds about$0.14/L to the price of oil. For palm oil sourcedbiodiesel to be competitive with petrodiesel, the priceof palm oil should not exceed $0.48/L, assuming anabsence of tax on biodiesel. Using the same analogy, areasonable target price for microalgal oil is $0.48/L foralgal diesel to be cost competitive with petrodiesel.Elimination of dependence on petroleum diesel andenvironmental sustainability require reducing the costof production of algal oil from about $2.80/L to $0.48/L. This is a strategic objective. The cost reductionnecessary declines to $0.72, if the algal biomass isproduced in photobioreactors and contains 70% oil byweight. These desired levels of cost reduction aresubstantial, but attainable.

Microalgal oils can potentially completely replacepetroleum as a source of hydrocarbon feedstock for thepetrochemical industry. For this to happen, microalgaloil will need to be sourced at a price that is roughlyrelated to the price of crude oil, as follows:

Calgal oil ¼ 6:9� 10−3Cpetroleum ð1Þ

where Calgal oil ($ per liter) is the price of microalgal oiland Cpetroleum is the price of crude oil in $ per barrel. Forexample, if the prevailing price of crude oil is $60/barrel,then microalgal oil should not cost more than about$0.41/L, if it is to substitute for crude oil. If the price ofcrude oil rises to $80/barrel as sometimes predicted, thenmicroalgal oil costing $0.55/L is likely to economicallysubstitute for crude petroleum. Eq. (1) assumes that algaloil has roughly 80% of the energy content of crudepetroleum.



Box 2

Light saturation and photoinhibition

Light saturation is characterized by a light satura-tion constant (Fig. B2), that is the intensity of light atwhich the specific biomass growth rate is half itsmaximum value, μmax. Light saturation constants formicroalgae tend to be much lower than the max-imum sunlight level that occurs at midday. Forexample, the light saturation constants for micro-algae Phaeodactylum tricornutum and Porphyridiumcruentum are 185 μE m−2 s−1 (Mann and Myers,1968) and ˜200 μE m−2 s−1 (Molina Grima et al.,2000), respectively. In comparison with thesevalues, the typical midday outdoor light intensity inequatorial regions is about 2000 μE m−2 s−1.Because of light saturation, the biomass growthrate is much lower than would be possible if lightsaturation value could be increased substantially.

Above a certain value of light intensity, a furtherincrease in light level actually reduces the biomassgrowth rate (Fig. B2). This phenomenon is known asphotoinhibition. Microalgae become photoinhibited atlight intensities only slightly greater than the lightlevel at which the specific growth rate peaks.Photoinhibition results from generally reversible da-mage to the photosynthetic apparatus, as a conse-quence of excessive light (Camacho Rubio et al.,2003). Elimination of photoinhibition or its postpone-ment to higher light intensities can greatly increasethe average daily growth rate of algal biomass.

Fig. B2. Effect of light intensity on specific growth rate ofmicroalgae.


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7. Improving economics of microalgal biodiesel

Cost of producing microalgal biodiesel can bereduced substantially by using a biorefinery based pro-duction strategy, improving capabilities of microalgaethrough genetic engineering and advances in engineer-ing of photobioreactors.

7.1. Biorefinery based production strategy

Like a petroleum refinery, a biorefinery uses everycomponent of the biomass raw material to produce use-able products. Because all components of the biomassare used, the overall cost of producing any given productis lowered. Integrated biorefineries are already beingoperated in Canada, the United States, and Germany forproducing biofuels and other products from crops suchas corn and soybean. This approach can be used toreduce the cost of making microalgal biodiesel.

In addition to oils, microalgal biomass containssignificant quantities of proteins, carbohydrates andother nutrients (Sánchez Mirón et al., 2003). Therefore,the residual biomass from biodiesel production process-es can be used potentially as animal feed (Fig. 6). Someof the residual biomass may be used to produce methaneby anaerobic digestion, for generating the electricalpower necessary for running the microalgal biomassproduction facility. Excess power could be sold todefray the cost of producing biodiesel.

Although the use of microalgal biomass directly toproduce methane by anaerobic digestion (Mata-Alvarezet al., 2000; Raven and Gregersen, 2007) is technicallyfeasible, it cannot compete with the many other low-costorganic substrates that are available for anaerobic digestion.Nevertheless, algal biomass residue remaining after theextraction of oil can be used potentially tomakemethane.Amicroalgal biorefinery can simultaneously produce biodie-sel, animal feed, biogas and electrical power (Fig. 6).Extraction of other high-value products may be feasible,depending on the specific microalgae used.

7.2. Enhancing algal biology

Genetic and metabolic engineering are likely tohave the greatest impact on improving the economics ofproduction of microalgal diesel (Roessler et al., 1994;Dunahay et al., 1996).Geneticmodification ofmicroalgaehas received little attention (León-Bañares et al., 2004).Molecular level engineering can be used to potentially:

1. increase photosynthetic efficiency to enable in-creased biomass yield on light;


2. enhance biomass growth rate;3. increase oil content in biomass;4. improve temperature tolerance to reduce the expense

of cooling;




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5. eliminate the light saturation phenomenon (Box 2) sothat growth continues to increase in response toincreasing light level;

6. reduce photoinhibition (Box 2) that actually reducesgrowth rate at midday light intensities that occur intemperate and tropical zones; and

7. reduce susceptibility to photooxidation that damagescells.

In addition, there is a need to identify possiblebiochemical triggers and environmental factors thatmight favor accumulation of oil. Stability of engineeredstrains and methods for achieving stable production inindustrial microbial processes are known to be impor-tant issues (Zhang et al., 1996), but have been barelyexamined for microalgae.

7.3. Photobioreactor engineering

Although a capability for reliable engineering andoperation of tubular photobioreactors has emerged(Acién Fernández et al., 1997, 1998, 2001; CamachoRubio et al., 1999; Molina Grima et al., 1999, 2000,2001; Sánchez Mirón et al., 1999, 2000; Janssen et al.,2003; Carvalho et al., 2006), problems remain.

Photobioreactor tubes operated with high-densityculture for attaining high productivity, inevitably con-tain a photolimited central dark zone and a relativelybetter lit peripheral zone (Molina Grima et al., 1999,2001). Light intensity in the photolimited zone is lowerthan the saturation light level (Box 2). Turbulence in thetube causes rapid cycling of the fluid between the lightand dark zones. The frequency of light–dark cyclingdepends on several factors, including the intensity ofturbulence, concentration of cells, optical properties ofthe culture, the diameter of the tube, and the externalirradiance level (Molina Grima et al., 2000, 2001).Under conditions of sufficient and excess external irra-diance, light–dark cycling of above a certain frequencycan increase biomass productivity relative to the casewhen the same quantity of light is supplied continuouslyover the same total exposure time (Philliphs and Myers,1953; Terry, 1986; Grobbelaar, 1994; Nedbal et al.,1996; Grobbelaar et al., 1996; Camacho Rubio et al.,2003). Light–dark cycling times of 10 ms, for example,are known to improve growth compared with continu-ous illumination of equal cumulative quantity. Benefi-cial effects of rapid light–dark cycling under lightsaturation conditions are associated with the short darkperiod allowing the photosynthetic apparatus of the cellsto fully recover from the excited state of the previousillumination event.


Various attempts have been made to estimate thefrequency of light–dark cycling (Molina Grima et al.,1999, 2000, 2001; Sánchez Mirón et al., 1999; Janssenet al., 2003; Richmond, 2004), but this problem remainsunresolved. Distinct from the productivity enhancingeffect of light–dark cycling, turbulence in a denseculture reduces photoinhibition and photolimitation byensuring that the algal cells do not reside continuously ineither the well lit zone or the dark zone for long periods.

In principle, motionless mixers installed insidephotobioreactor tubes can be used to substantiallyenhance the mixing between the peripheral lit zoneand the interior dark zone (Molina Grima et al., 1999,2001; Sánchez Mirón et al., 1999). Such mixers haveproved useful in other tubular reactors (Chisti et al.,1990; Chisti, 1998; Thakur et al., 2003). Unfortunately,existing designs of motionless mixers are not satisfac-tory for photobioreactors because they substantiallyreduce penetration of light in the tubes. New designs ofmotionless mixers are needed.

Like cells of higher plants (Moo-Young and Chisti,1988) and animals (Zhang et al., 1995; Chisti, 2000,2001; García Camacho et al., 2005), microalgae aredamaged by intense hydrodynamic shear fields thatoccur in high-velocity flow in pipes, pumps and mixingtanks (Chisti, 1999a; García Camacho et al., 2001, 2007;Sánchez Mirón et al., 2003; Mazzuca Sobczuk et al.,2006). Some algae are more sensitive to shear damagethan others. Shear sensitivity can pose a significantproblem as the intensity of turbulence needed inphotobioreactors to generate optimal light–dark cycling(Grobbelaar et al., 1996; Camacho Rubio et al., 2003) isdifficult to achieve (Molina Grima et al., 2000, 2001;Camacho Rubio et al., 2004) without damaging algalcells. Methods have been developed to reduce thedamage associated with turbulence of limited intensity(García Camacho et al., 2001; Mazzuca Sobczuk et al.,2006). Intensities of shear stress are not easilydetermined in bioreactors (Chisti and Moo-Young,1989; Chisti, 1989, 1999a), but improved methods fordoing so are emerging (Sánchez Pérez et al., 2006).

Some algae will preferentially grow attached to theinternal wall of the photobioreactor tube, thus preventinglight penetration into the tube and reducing bioreactorproductivity. Robust methods for controlling wall growthare needed. Wall growth is controlled by some of thefollowing methods: 1. use of large slugs of air tointermittently scour the internal surface of the tube;2. circulation of close fitting balls in continuous run tubesto clean the internal surface; 3. highly turbulent flow; and4. suspended sand or grit particles to abrade any biomassadhering to the internal surface. Potentially, enzymes that




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digest the polymer glue that binds algal cells to the tubewalls, may be used for controlling wall growth.

Bioprocess intensification approaches (Chisti andMoo-Young, 1996; Chisti, 2003) that have proved sosuccessful in improving the economics of various bio-technology based processes have been barely assessed foruse with photobioreactors.

8. Conclusion

As demonstrated here, microalgal biodiesel is techni-cally feasible. It is the only renewable biodiesel that canpotentially completely displace liquid fuels derived frompetroleum. Economics of producing microalgal biodieselneed to improve substantially to make it competitive withpetrodiesel, but the level of improvement necessaryappears to be attainable. Producing low-cost microalgalbiodiesel requires primarily improvements to algalbiology through genetic and metabolic engineering. Useof the biorefinery concept and advances in photobior-eactor engineering will further lower the cost ofproduction. In view of their much greater productivitythan raceways, tubular photobioreactors are likely to beused in producing much of the microalgal biomassrequired for making biodiesel. Photobioreactors provide acontrolled environment that can be tailored to the specificdemands of highly productive microalgae to attain aconsistently good annual yield of oil.

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